The general objective of this proposal is to elucidate the mechanisms by which the interaction of actin and myosin underlying contraction is regulated in vascular smooth muscle. This proposal is intended to study the role of myosin light chain phosphorylation, Ca2+ binding to myosin, and leiotonins on the regulation of actin-activated ATPase activity of myosin isolated from arterial and visceral smooth muscles. The exitence of a dual CA2+ regulation, one mediated my myosin and the other mediated by CA2+ binding regulatory proteins (e.g. leiotonins) located in the thin filament, will be investigated. The role of 20,000 dalton light chain on the regulation of actomyosin ATPase will be studied using myosin deficient in light chain and by exchanging light chain between myosin that does not exhibit direct myosin mediated calcium sensitivity after phosphorylection (e.g. gizzard myosin, stomach muscle myosin) and myosin that does exhibit calcium sensitivitiy (e.g vas deferens myosin, arterial myosin). Furthermore; the role of tropomyosin on the regulation of actin-activated myosin ATPase activity will be determined using HMM and S-1 prepared from smooth muscle.